Syntax updater

[nittka]

This PR addresses #128. The invocation of the converter needs to be done differently on windows, where .py files may not necessarily be treated as executable. The implemented way looks for a python-executable next to the LilyPond executable. If python did not come with LilyPond it is expected to be on the path. (A more elaborate version would ask for the python executable path).

With your permission, I would add two further modifications to this PR.

• I would invoke the syntax update only if Lilypond compilation is activated as well. To me it makes no sense to continuously run the syntax updater when editing and saving, if I don't want to compile as well.
• I would like to remove the redundant --edit parameter from SyntaxUpdaterProcessBuilderFactory.get, this parameter is set in CompilerJob anyway (whether the file is opened in an editor or not).

[nittka]

With input from http://lilypond.org/doc/v2.19/Documentation/usage/updating-files-with-convert_002dly I could make convert-ly actually do something on my machine. With the PR, a version 2.16.0 is turned into 2.18.2 - whether the file is open or not when compiling.